1. Field of the Invention
The present invention relates to a tandem color image forming apparatus using a plurality of image carriers and a color image forming method. In particular, the present invention relates to a color image forming apparatus capable of effectively suppressing the generation of black spots even when a continuous image forming process is performed, and to a method of forming a color image using the same.
2. Description of the Related Art
In recent years, tandem color image forming apparatuses capable of forming high-quality color images at a high speed have come into widespread use.
The tandem color image forming apparatus includes a plurality of image forming units having image carriers using for each color developer, and in the image forming units, developer images using for each color developer are formed on the image carriers. Then, the developer images using for each color developer are overlapped on a recording material or an intermediate transfer body, thereby forming a color image.
Further, in the tandem color image forming apparatus, an organic photoconductor has been widely used as the image carrier since it has a high charging property and can stably support a developer even when an image forming process is performed at a high speed.
Meanwhile, when the organic photoconductor is used, an organic photoconductor using for a black developer is more likely to be worn away than organic photoconductors using for the other color developers since monochrome printing is frequently performed in the color image forming apparatus.
In order to solve the above problem, a color image forming apparatus has been disclosed in which only the image carrier using for the black developer is composed of an amorphous silicon photoconductor having high abrasion resistance (for example, see JP10-333393A).
More specifically, a color image forming apparatus has been disclosed in which only the image carrier using for the black developer is composed of an amorphous silicon photoconductor including an amorphous silicon carbide photoconductive layer with a predetermined thickness, organic photoconductors are used as image carriers using for the other color developers, and a difference between the charging potential of the amorphous silicon photoconductor and the charging potential of the organic photoconductors is 200 V or less.
However, when the amorphous silicon photoconductor disclosed in JP10-333393A is used, it is difficult to compensate for a reduction in the charging property. In addition to the charging property, since the amorphous silicon photoconductor has different exposure characteristics or transfer characteristics from the organic photoconductors, it is complicated and difficult to individually control these characteristics.
In addition, a color image forming apparatus has been disclosed which uses an organic photoconductor as the image carrier using for the black developer to improve the abrasion resistance thereof (for example, see JP2001-51467A).
More specifically, a color image forming apparatus has been disclosed in which a non-contact charging method is used for only the organic photoconductor using for the black developer, a photosensitive layer of the organic photoconductor using for the black developer is formed to have a relatively large thickness, or a binding resin of the photosensitive layer has a relatively large viscosity average molecular weight.
When the color image forming apparatus disclosed in JP2001-51467A is used, it is possible to improve the abrasion resistance of the photosensitive layer of the organic photoconductor using for the black developer. However, when a continuous image forming process is performed, filming is likely to occur in the organic photoconductor using for the black developer, which may cause black spots to be generated on a formed image.
More specifically, since the organic photoconductor using for the black developer is more frequently used than organic photoconductors using for the other color developers and is generally arranged in the vicinity of a position where paper powder is generated, such as a transfer unit or a fixing unit, remaining toner or paper powder is likely to be adhered to the surface of the photosensitive layer, which may cause filming.
In particular, when a non-magnetic single-component developer produced by polymerization method is used as the developer and a cleaner-less type without a blade cleaner is adopted to perform a continuous image forming process, it is more difficult to suppress the occurrence of filming and the generation of black spots on a formed image in the organic photoconductor using for the black developer.
The inventors found that it was possible to effectively suppress the generation of black spots on a formed image, even when filming occurred in the image carrier, by setting the resistance per unit area of a base body of an image carrier using for at least a black developer, or the resistance per unit area of the base body through an intermediate layer in a predetermined range, in a tandem color image forming apparatus. The present invention has been made on the basis of the findings.